The theory has its origins in the dual resonance model (1969). Since that time, the term string theory has developed to incorporate any of a group of related superstring theories. Five major string theories were formulated. The main differences between each of them were the number of dimensions in which the strings developed and their characteristics, however all appeared to be correct. In the mid 1990s a unification of all previous superstring theories, called M-theory, was proposed, which asserted that strings are really 1-dimensional slices of a 2-dimensional membrane vibrating in 11-dimensional space.